THE TIME EVOLUTION OF THE HARD AND SOFT ACIDS AND BASES THEORY - Theoretical and Computational Research in the 21st Century - page 51

where the dominant interaction between the acidic and basic re-
agents is not the interaction between their reactive sites, the Ayers
model based on the reactive-site interaction energy will not be
valid.
The single-exchange reactions are much more insightful to differences
in the intrinsic strength of the acids and bases. Ayers made the conclusion
that the HSAB principle is much less robust for single-exchange reactions
and isomerization reactions of ambidentate ligands.
This is due to the fact that in single-exchange reactions electron-trans-
fer effects and electrostatic effects favor different products. The double-
exchange reaction follows the HSAB principle nicely because of the fact
that in such case the electron-transfer effects and electrostatic effects rein-
force each other.
For example, if electrostatic effects are more important than electron
transfer effects in the acid-exchange reaction with a soft base, Eq. (78),
then the reaction will favor the hard acid-soft base product. However, the
electron-transfer effects will ordinarily dominate in the case of the soft
reagent because the reagents are associated with small charges and large
amounts of electron transfer.
Ayers proved that the HSAB principle is a driven by simple electron
transfer effects and mathematically showed [73] that “for the exchange
reaction, wherein two molecules, one the product of reacting a hard acid
and a soft base and the other the product of reacting a soft acid with a hard
base, exchange substituents to form the preferred hard-hard and soft-soft
product.”
It is very important to remember that the HSAB principle only ap-
plies when the strengths of acids and/or bases are similar/closer, and these
exceptions to the HSAB principle indicate interesting molecule-specifi c
properties that are not directly related to the chemical hardness of the re-
agents.
1.2.21 LIMITATION OF THE HOMO-LUMO CONCEPT OF
CHEMICAL INTERACTION—THE FERMO CONCEPT TO THE
HSAB PRINCIPLE
da Silva et al. [74, 75] opined that the HOMO-LUMO gap itself cannot
describe the hardness difference between the various binding sites in the
same molecules such as those formed by ambidentate ligands, such as